1. Field of the Invention
The present invention relates to a projection display apparatus for projecting and displaying an image using a liquid crystal panel, especially to a mechanism and a method for cooling the liquid crystal panel and polarizing plates.
2. Description of the Related Art
In a LCD (Liquid Crystal Display) projector apparatus, which is a projection display apparatus, the quality of projected images is rapidly improving by virtue of the improved luminous efficiency of a lamp, the increased density and aperture ratio of light valves, and improvements in illuminating optical system. Therefore, LCD projectors are used in wide applications from home theater use to business presentation use.
Referring to FIG. 1, explanations are given about the basic configuration of a conventional LCD projector. An LCD projector according to the present invention also has the same configuration as will be explained below. Therefore, the same reference numerals are used for the elements that are common in the present invention. LCD projector 1 includes illuminating optical system 2, color separating optical system 7, and focusing optical system 12 as basic systems.
Illuminating optical system 2 is provided with light source 3 including a high-luminance lamp, such as an ultra high pressure mercury lamp, reflector 4 to reflect light that is emitted from light source 3, light integrators 5a, 5b to obtain an uniform illumination distribution of the light that is reflected by reflector 4, polarizing beam splitter (PBS) 6 to convert randomly polarized light into linearly polarized light, and field lens 11a. 
Color separating optical system 7, which is positioned downstream relative to illuminating optical system 2, is provided with field lens 11b, dichroic mirrors 8a, 8b, reflective mirrors 9a, 9b, 9c, 9d, and relay lenses 10a, 10b. Dichroic mirrors 8a, 8b separate light that comes from illuminating optical system 2 into color bands of red (R), green (G), and blue (B), and make the color bands incident on the corresponding liquid crystal panels.
Focusing optical system 12, which is positioned downstream relative to color separating optical system 7, is provided with optical modulation portion 13 to modulate each color band that is emitted from color separating optical system 7 in accordance with given image information, color combining prism 14 to combine the color bands that are modulated, and projection lens 15 to project the combined light onto a screen.
Optical modulation portion 13 is provided with three liquid crystal panels 16a, 16b, 16c, which are translucent display devices, incident side polarizing plates 17a, 17b, 17c which are arranged on the incident sides of the respective liquid crystal panels, and emitting side polarizing plates 18a, 18b, 18c which are arranged on the emitting sides of the respective liquid crystal panels.
Since a TN (Twisted Nematic) liquid crystal panel can use only a light component that is polarized linearly in a specific direction, each color band that is emitted from color separating optical system 7 is polarized in a predetermined direction by incident side polarizing plates 17a, 17b, 17c (P polarization). P-polarized color bands are modulated by liquid crystal panels 16a, 16b, 16c, and only S-polarized components of the modulated color bands pass through emitting side polarizing plates 18a, 18b, 18c. 
The above-described optical system is applied to a three-panel LCD projector in which light that comes from a lamp is separated into three primary color bands, and the separated color bands are modulated by three liquid crystal panels, respectively. However, an optical modulation unit having a similar configuration can also be applied to a single-panel LCD projector which has single liquid crystal panel and which is less bright and less expensive.
In such optical modulation portion 13, incident side polarizing plates 17a, 17b, 17c and emitting side polarizing plates 18a, 18b, 18c tend to be heated by heat absorption, because each plate transmits only the light beam that is polarized in one direction and shields the remaining light beams. Further, liquid crystal panels 16a, 16b, 16c generate heat during operation because black matrixes, which are arranged between pixels, shield the light beams.
Liquid crystal panels and polarizing plates are often made of organic materials. Therefore, an orientation layer of the panel may be damaged or the polarization characteristic of the polarizing plate may be degraded when the liquid crystal panels and the polarizing plates are irradiated by light having short wavelengths for a long period, or are exposed under high temperature conditions, resulting in a serious loss of the functions. This phenomenon may shorten the life time of the product, and raise maintenance cost due to the replacement of the unit. Further, the quality of the combined projection image may be degraded because of the fluctuation in the properties of each color band. For these reasons, there exist a need for measures against heating of an optical modulation unit.
Now, conventional methods for cooling incident side polarizing plates, emitting side polarizing plates, and liquid crystal panels to suppress temperature rise will be briefly described. In this specification, the combination of an incident side polarizing plate, an emitting side polarizing plate, and a liquid crystal panel is called a liquid crystal unit. The combination of three liquid crystal units and a color combining prism is called a liquid crystal unit assembly.
Referring to FIGS. 2A and 2B, cooling air 124 is supplied by fan 120 via an intake, not shown, which is provided on the housing of a LCD projector. The air is conveyed to duct openings 122a, 122b, 122c via the exhaust of the fan and duct 121. Duct openings 122a, 122b, 122c are arranged just below corresponding liquid crystal units 119 of liquid crystal unit assembly 123.
Incident side polarizing plate 117, liquid crystal panel 116, and emitting side polarizing plate 118, which together constitute each of three liquid crystal units 119, are spaced with gaps. Cooling air 124 passes through the gaps from bottom to top, and heat is removed from the surfaces through forced convection heat transfer.
A mechanism for cooling a liquid crystal unit using a coolant tank is disclosed in Japanese Patent Laid-Open Publication No. 2002-287244 (Patent Document 1) and No. 2003-195253 (Patent Document 2). Referring to FIG. 3, liquid coolant 233 that is filled in coolant tank 243 is held between emitting side polarizing plate 218 of liquid crystal unit assembly 223 and seal plate 244. This prior art is characterized such that coolant tank 243 is fixed to the side wall of holders 228 for color combining prism 214. Referring to FIG. 4, liquid coolant is filled in coolant tank 343 of liquid crystal unit assembly 323. This prior art is characterized such that liquid crystal panels 316 is also cooled in addition to emitting side polarizing plates.
Japanese Patent Laid-Open Publication No. 160793/99 (Patent Document 3) discloses a mechanism, which has a heat sink that is mounted in a duct, for cooling liquid crystal units. Referring to FIG. 5, liquid crystal unit assembly 423 that includes incident side polarizing plates, liquid crystal panels, and emitting side polarizing plates is cooled by forced air cooling. Cooling air flows in an airtight closed duct, and heat sink 438 to cool the air is arranged in duct 446.
Japanese Patent Laid-Open Publication No. 2000-269674 (Patent Document 4) discloses a mechanism for cooling liquid crystal units in which a heat sink that is connected with a Peltier element is arranged in a duct. Referring to FIG. 6, first heat sink 538a for receiving heat, which is connected with Peltier element 539, is inserted into duct 521 that is connected to cooling fan 520. Second heat sink 538b for discharging heat is connected to a heat-discharging side of Peltier element 539. Since air that is pre-cooled to a lower temperature is supplied to the liquid crystal unit, not shown, the cooling efficiency can be enhanced.
However, there are the following drawbacks in the prior art techniques. In accordance with the increasing need for a reduction in size and an increase in brightness of LCD projectors, lamp output has been increased and the size of display devices has been reduced. As a result, the heat load has been increased due to the increased density of luminous flux that is incident on a liquid crystal unit.
For example, a LCD projector (1.0 type-XGA) in 2000 Im class has a total heat generation of approximately 15 W in liquid crystal units, and a heat flux of approximately 0.6 W/cm2 in an emitting side polarizing plate. On the other hand, a LCD projector in 5000 lm class has a heat generation of approximately 35 W or more in liquid crystal units, and a heat flux of approximately 1.4 W/cm2 or more in an emitting side polarizing plate.
In the prior art using forced air cooling, which was explained with reference to FIGS. 2A and 2B, the flow rate of supply air is increased in order to cope with the increased heat load. The flow velocity of the cooling air around a heat source is increased, and a larger amount of heat can be removed from the heat source due to the improved heat transfer efficiency and the improved cooling capability.
In order to increase the flow rate of supply air that is supplied by a fan, a fan with a larger diameter or with a higher speed is used. In an application to cool liquid crystal units, a fan with a higher speed is used taking into consideration the structural limitation for mounting a fan. However, the increase in the number of revolutions of a fan directly causes an increase in noise, which impairs comfortable operation for users and also causes degradation in commercial value. This is the reason why various measures have been taken, as described in Patent Documents 1 to 4.
According to the prior arts that are disclosed in Patent Documents 1 and 2, liquid coolant that is filled into coolant tanks 243, 343 is in contact with an emitting side polarizing plate or liquid crystal panel 316, and heat is removed via the contact area through heat diffusion. However, these prior arts have the following problems.
The first problem is the quality of images. Since liquid coolant is filled between the optical elements of the liquid crystal unit, light passes through a liquid layer that exists between the optical elements. For example, referring to Patent Document 2, there is a liquid layer between the liquid crystal panel and the emitting side polarizing plate. Such a liquid layer disturbs the polarization of light when the light passes through the liquid layer due to the generation of bubbles, fluctuations in the coolant density that is caused by heat transportation, and convection-induced thermal fluctuations that is caused by the fluctuations in the coolant density. As a result, image information that passes through emitting side polarizing plate 118 is disturbed, resulting in degraded quality of projected images.
The second problem is reliability and mountability. In a liquid cooling system for removing heat from optical elements in liquid crystal unit assembly 223, 323, the liquid coolant expands and contracts repeatedly depending on the operation condition. Therefore, the coolant tank has a pressure regulating mechanism to accommodate the expansion and contraction. However, there is a risk of leakage of coolant due to failure of the pressure regulating mechanism after long use. Additionally, the complicated sealing mechanism and pressure regulating mechanism of coolant tanks 243, 343 make the assembling process of the liquid crystal unit more difficult.